This invention relates generally to railroad car suspension systems, and more particularly relates to a novel isolator pad placed between each of the railroad car axle roller bearing adaptors and the car truck sideframes, which effectively decrease the unsprung mass of the car and make it possible to increase the payload of the car without causing an increase in damage to rails and roadbed.
Intermodal wellcars for carrying containers have in the past used 100 ton trucks having 36" wheels. Each such car has a total load capacity of 131,500 lbs. which when reduced by the weight of the car itself provides carrying capacity for containers of approximately 100,000 lbs. Containers are usually on the order of 45 feet long and are carried with one container in the well and the second container stacked above it. The containers, on average, weigh between 50,000 and 60,000 lbs. each. Accordingly, in order to double stack containers, even 50,000 lb. containers constitute a marginal load when double stacked in a wellcar, and it is not possible to stack two 60,000 or a 50,000 and a 60,000 lb. container in a wellcar without overloading the trucks.
There is, however, available for use, a truck which is designated as a 125 ton truck and has 38" wheels. A wellcar using a pair of trucks of this type has a load carrying capacity of 157,500 lbs. which when reduced by the weight of car itself leaves a load carrying capability of approximately 125,000 lbs. This allows for double stacking of perhaps 99% of all containers in use today. The problem with the 125 ton trucks is that the railroads have not wanted to use them because they produce excessive track and roadbed damage as compared to the 100 ton truck. Therefore, if it is possible to build a 125 ton truck which has dynamic characteristics on the rail and roadbed which are approximately those of the 100 ton truck, the load carrying capability of the wellcar can be materially increased without producing the adverse effects on the rails and roadbed, and accordingly would be acceptable by the railroad industry.
The prior patented art includes U.S. Pat. 3,381,629 to Walter B. Jones entitled "Cushion Mounted Bearing Adaptor For Railway Trucks" which discloses a pad used in the same location but for a different purpose, namely, as set forth in the Jones specification (column 1) "a resilient element over each bearing assembly which serves to accommodate lateral movements between the bearing assemblies and the truck frames to reduce and substantially eliminate lateral shocks to the side frames resulting from "hunting" of the wheels."
As will be subsequently described in connection with FIG. 4 of the drawings, labeled "PRIOR ART", Jones' device consists of a resilient pad sandwiched between and bonded to an upper steel plate and a lower steel plate, the resilient pad being specified as "rubber or synthetic rubber or any suitable plastic material". Rubber and synthetic rubber can not be so used because in use they are extruded outward from between the steel plates and quickly become ineffective. It is known that Jones type pads utilizing rubber have been tried in the past, in 100 ton truck cars, and have failed after very short use with cars which were substantially underloaded. The use of such devices was abandoned by the railroads before the advent of the intermodal double stack wellcars.
Because of the pressing nature of the need to increase the load carrying capacity of wellcars to handle double stacked 60,000 lbs. containers, and thereby substantially increase the economies of rail transportation, the feasibility of using 125 ton trucks was reconsidered. The accelerated rail wear problems and roadbed damage considerations normally associated with the use of such trucks precluded acceptance of such use unless someway could be devised to prevent such consequences. Accordingly, a vertical isolator pad was developed of the type to be subsequently described in connection with FIG. 3, consisting of a polyether based urethane elastomer pad having steel facing sheets bonded to the upper and lower surfaces. These isolator pads were extensively tested at the American Association of Railroads Transportation Test Center in Pueblo, Colo. with instrumented track and instrumented wheel sets on several different kinds of track situations. The results of these tests are shown in FIGS. 5 through 11 to be subsequently described.
In summary, these tests indicated that the articulated geometry and the primary suspension system of the vertical isolator pads in the 125 ton articulated wellcars are effective to reduce both vertical and lateral dynamic forces to magnitudes that do not exceed comparable car forces generated by 100 ton cars. In many cases the articulated wellcars produced much lower forces than the 100 ton equipment, showing that the vertical isolator pad does, in fact, reduce dynamic forces on 125 ton four wheel trucks. Since these lower forces lower rail contact and rail bending stresses, the vertical isolator pad, in combination with articulated cars, permits the use of higher axle capacities without adversely affecting rail or support structures.
The only remaining question was how the pads would perform under actual operating conditions in normal railroad service. To determine this, a number of cars were fitted with the vertical isolator pads and put into service in various parts of the country to experience varying weather and environmental conditions, and data was accumulated. After these pads were in experimental use for approximately a year to fifteen months failures began to appear. One type of failure was the separation of the steel face plate from the elastomer pad. The second type of failure was a compressive failure of the elastomer, which appeared as a flattening and a partial extrusion of the elastomer out from between the steel plates. This resulted in substantial degradation of the resilient performance of the pad. The degradation was such that ultimately the pad had no impact reducing effect whatever. A third adverse consequence occurred when the pads had been degraded, which was the creeping of the pad structure up out of the pocket on the roller bearing adapter, which led to uneven loading on the roller bearing and ultimate failure of the bearing.
Extensive testing was then undertaken to attempt to determine why these pads were failing. The compressive failure problem was laboratory tested by subjecting the pads to compressive forces substantially three times their normal operating load of 38,000 pounds, or 114,000 pounds loading on the pad. These tests showed no evidence whatever of compressive failure, and no permanent set of the pad when the vertical loading was released. The failed pads were reexamined, and it appeared that there was some evidence that the elastomer had been subjected to excessively high temperatures, much higher than would be encountered by being used in hot environments. The normal temperatures encountered by the pad in its environment would be on the order of 150.degree. Fahrenheit just due to heat generated by the bearing. The type of heat condition that was evidence by the failed pads was far in excess of 150.degree., and such pad failures occurred even When the bearings were in perfectly proper operating condition. The pads were then heated in ovens to 150.degree. and some to 200.degree. Fahrenheit and then the static three times compressive load test was repeated. In no case was there any pad failure. The problem was still not understood and yet other compressive test runs in which the vertical loading forces were raised to 400,000 pounds per pad under static conditions at room temperature did not produce pad failure. A dynamic test was performed in which the pad was subjected to a compressive cyclicly changing loading. The initial loading was set at 38,000 pounds, the normal loading for the pad, and the pad was then subjected compressively to a triangular waveform which increased to 68,000 pounds and then reduced to 38,000 pounds continuously at a four Hertz rate. This was done to determine whether the variation in loading which produced some flexing of the pad could in fact generate internal heat, and was a conservative test in that it overstressed the pad, because actual in-use testing determined that the cyclic rate applied to the pads in actual use is on the order of two Hertz. Additionally, the 68,000 pounds peak load was selected on the basis of being the maximum impulse load that the pad would be subjected to in actual use. The tests were run on each pad for at least one million cycles, and it was determined that the internal temperature rise in the pad was not in excess of 20.degree. Fahrenheit. The pads tested showed no evidence of failure whatever. The one million cycle test was considered to represent between two and three years of actual service in the field.
It subsequently became known that the temperature data supplied by the railroads was in error in that it had been indicated that the railroads overheated bearing detectors would be actuated at 200.degree. Fahrenheit. In fact, the 200.degree. Fahrenheit temperature was not actual temperature, but 200.degree. Fahrenheit above ambient. The ambient temperature in a desert summer condition could itself be at 120.degree., thus giving a detected actual temperature of 320.degree. Fahrenheit. None of the testing had been done at these temperatures, so that all of the previous data based upon temperature had to be reconsidered. The previous tests were then duplicated at 250.degree. Fahrenheit, 300.degree. Fahrenheit and 350.degree. Fahrenheit and showed some rather different results from the previous tests. At 250.degree. Fahrenheit the pads performed well. At 300.degree. Fahrenheit there began to be some evidence of the pads starting to take a set. At 350.degree. Fahrenheit the set became much more pronounced, and it was considered that in actual service this condition would lead to a pad failure. This was the first indication that a high temperature elastomer was required.
One additional observation of the pads became highly significant, namely, that scoring was observed on the outer surfaces of the steel plates, indicating that relative motion had taken place laterally between the pad and the surfaces of the wheel bearing adapter. This suggested that additional heat might have been generated by the frictional engagement, and that the high heat build up in the steel plates due to this frictional engagement eventually caused the separation of the plates from the elastomer pad. Since the elastomer has a melting point in the 500.degree. Fahrenheit range it became evident that the elastomer at the inner face with the steel plate had been subjected to temperatures in that range in order to cause separation. This significant information led to the consideration of several modifications to the vertical isolator pad.
The first change was to make a pad without the steel plates so that there would not be the heat sink effect of the steel plates reaching a high temperature and creating failure of the bond. Additionally, in order to eliminate scoring motion that had appeared on the steel plates the dimensions of the pad were increased so that it fit snugly in the pocket of the bearing adaptor, and accordingly could not shift laterally within the pocket. Also, because of the determination of the temperatures which could be achieved at the surfaces of the pad it was necessary to utilize a higher temperature elastomer which would have a melting range somewhere between 550.degree. and 650.degree. Fahrenheit. It was also observed that once the steel plates came loose from the elastomer pads, the movement of the plates relative to the pad would chew up the entire surface of the elastomer, and ultimately the plates destroyed the pad. This information flew directly in the face of a specification that had been set by the railroads, which was that unless the pads had steel faces so that there would be a steel to steel contact in the use environment, the railroad industry would not consider using such a pad. Accordingly, it was a requirement set by the railroads which unknowingly was a key factor in the failure of the pads.
From the foregoing information, a new vertical isolator pad according to the invention was conceived, of the configuration shown in FIG. 2, and to be subsequently described. The material selected has a Shore hardness durometer of about 65 and is marketing by Air products and Chemicals, Inc. under its trademark polathane XPE System-30 High-performance Urethane. The pad was made thicker so that the height of the elastomer was equal to the height of the composite original pad, which had been elastomer plus two sheets of steel facing. The bottom portion of the pad was molded of rectangular cross section so that it would fit exactly within the pocket, and the portion of the pad that extended above the surface of the pocket edges was tapered inwardly so that it formed a trapezoidal cross section. This tapering is necessary because under load conditions the portion of the pad not retained within the pocket tends to bulge laterally, and bulging with straight pad sidewalls could exert vertical forces tending to cause the pad to migrate out of the pocket, which would cause failures similar to those previously encountered due to unequal loading of the bearing adapter. The newly devised pad was retested at 250.degree., 300.degree. and 350.degree. Fahrenheit under the three times static load of 114,000 pounds. The results showed that the pads did not take any permanent set under any of these conditions, indicating that these pads were far more temperature resistent than the previous pads and would not be subject to failures of the kind encountered during the use tests. The pads according to the invention were also dynamically tested, as Will be subsequently described in connection with FIGS. 5, 6 and 14, with the result that the useful life of these pads is projected at one million miles of railroad car service corresponding to substantially three to five years of actual car usage, and meeting the requirements of the railroad industry.